French Patent Specifications Nos. 1 438 330, 2 105 710 and 2 146 612 already describe the production of a sealed insulating vessel forming part of the supporting structure of a ship and consisting of two successive sealing barriers, a primary sealing barrier in contact with the liquefied gas being transported and a secondary sealing barrier disposed between the primary barrier and the supporting structure of the ship, these two sealing barriers alternating with two thermally insulating layers referred to as "insulating barriers". In these embodiments, the primary and secondary insulating barriers consist of parallelepipedal tanks filled with a particular insulant and the primary and secondary sealing barriers consist of metal strakes, e.g. made of invar, welded via their bent over edges to either side of a welding flange.
French Patent Specification No. 2 462 336 proposes an embodiment of a vessel in which the secondary insulating barrier is formed by a thick layer of cellular material fixed to the supporting structure of the ship, the primary insulating barrier consisting of a rigid plate having an advantage with respect, inter alia, to mechanical resistance, as the rigidity of the plates of the primary insulating barrier allows for improved resistance with respect to the shocks produced at the walls of the vessel by the movements of the liquid being transported, these movements being the result of the roll and pitch of the ship. In this embodiment, the primary barrier is coupled to the secondary barrier without any connection to the supporting structure of the ship, this being very advantageous with respect to the insulating properties. However, the essential disadvantage is that automated construction is virtually impossible, so that the manufacturing price proves prohibitive, in spite of the good results obtained. Moreover, at the primary insulating barrier, a sealed bulkhead is created between two adjacent elements of the said barrier, making it very difficult to purify the primary barrier by the circulation of inert gas or to monitor the seal by the injection of tracer gas.
French Patent Specification No. 2 504 882 proposes an embodiment of a vessel of this kind in which the secondary insulating barrier consists, of parallelepipedal tanks filled with insulant and the primary insulating barrier consists of plates formed by a cellular layer fitted to a rigid panel. This type of structure has the advantage that it retains the essential advantage of the rigidity of the primary insulating barrier as proposed in the aforesaid Patent Specification No. 2 462 336. Unfortunately, this device also has a serious disadvantage, as the primary barrier is coupled directly to the supporting structure of the ship by means of anchoring members which traverse the secondary sealing barrier. It has been found that under certain conditions this technique is capable of producing zones of concentrated stress, this being disadvantageous with respect to safety. In addition, the anchoring members establish a direct thermal bridge between the primary barrier and the supporting structure of the ship, this being very disadvantageous with respect to the insulating capacity.
French Patent Specification No. 2 629 897 proposes an embodiment of a vessel of this kind in which, on the one hand, rigid plates providing good mechanical resistance to shocks from the liquid being transported are used as an element of the primary insulating barrier and, on the other hand, no direct thermal bridge is created between the primary barrier and the supporting structure of the ship, and, finally, mounting can be achieved by automatic mounting means, reducing the manufacturing cost of the vessel. This embodiment uses a secondary insulating barrier consisting in the known manner of rigid tanks filled with a particular insulating material. The secondary sealing barrier consists of invar strakes welded via their bent over edges to either side of a weld support held on the tanks of the secondary insulating barrier and this same weld support serves to hold the elements of the primary insulating barrier. However, this embodiment has a disadvantage as a result of the fact that the elements of the secondary insulating barrier are fixed via their corners and that the tensile forces exerted on the weld supports are applied in zones remote from the fixing corners, which may result in dynamic deformation of the elements of the secondary insulating barrier being used. Moreover, the primary barrier is coupled exclusively by means of a weld support held by a right-angled fold to the face of the tanks of the secondary insulating barrier which supports the secondary sealing barrier. This method of operation does not give the degree of tear resistance desired for maximum safety and, in addition, it makes it necessary to ensure a screw connection between the face which supports the coupling and the reinforced internal bulkhead situated at right angles with the said coupling, resulting in a not inconsiderable increase in the cost price. Finally, as in the aforesaid prior embodiments, one single retaining member cooperates with four adjacent elements, making it difficult to mount.